The present invention relates to an apparatus and method for heating articles and for measuring the temperature of the heated articles. The invention is particularly useful for rapid thermal processing of semiconductor wafers and is therefore described below with respect to this application.
Temperature measurement and temperature uniformity are major difficulties in thermally processing semiconductor wafers by the Rapid Thermal Process (RTP). RTP is used in many thermal processes to optimize the heating of the wafer to reduce cycle time, and to enable fast processing. The wafer is held in a reactor (reaction chamber) and heated by lamps, generally (but not necessarily) tungsten-halogen lamps. The heating rate may exceed 100.degree. C./sec, as compared to 1-2.degree. C./minute in regular furnaces, or 10-50.degree. C./minute in mini-furnaces. The temperature of the chamber walls and other components near the wafer may be totally different (usually lower) than the wafer temperature. The temperature of the reactor gas, if present, may also be different from the wafer temperature.
The physical and chemical reactions that are used to produce electronic devices on semiconductor wafers are very sensitive to temperature. To produce the desired reactions, the temperature over the entire wafer must be uniform within narrow tolerances.
The wafer temperature can not be detected by a device that contacts the wafer because the heat transfer is too slow from the wafer to the temperature detecting device; also, such contact may affect the temperature uniformity over the complete surface of the wafer. Therefore, the temperature is calculated from wafer radiation, measured by a pyrometer. The radiation e of a body at temperature T is given as a function of the wavelength .lambda. as follows: ##EQU1## where .epsilon. is the emissivity of the body, which depends on the material and surface conditions. The value of the emissivity is always lower than unity. For an opaque body (a body that does not transmit light) the sum of the emissivity and the reflectivity P is 1; that is: EQU .epsilon.(T,.lambda.)+p(T,.lambda.)=1 (2)
For example, if a body reflects 80% (0.8) from the light at a specific temperature and wavelength, its emissivity is 0.2.
If the emissivity of the measured body is known or is constant, calibration and temperature measurement of the body become simple. Unfortunately, this is not the case with semiconductor wafers used for production. During production, layers are added, removed or change many times. The emissivity of a given wafer may vary widely in the range of 0.19-0.91, during the process and is not known beforehand. Direct calculation of the temperature from calibration curves of a standard wafer gives poor results.
The wafer apparent emissivity may be enhanced. A known enhanced emissivity method for temperature measurement uses a flat reflector located at and spaced from the back side of the wafer. The light emitted from the wafer is reflected by the reflection and returns to the wafer, where part of it is reflected back to the reflector and so on. Low emissivity wafers reflect a larger part of the light. The total radiation flux to the mirror is the sum of all the repeated reflections. The simplest model assumes that the wafer and the mirror are two infinite parallel plates, and the measuring device has negligible influence (see below). The enhanced emissivity .lambda..sub.eff is ##EQU2## where the subscripts w and P are for the wafer and the reflector, respectively.
The radiation emitted by the wafer is generally detected by an optical fiber having one end exposed to the space between the backside of the wafer and reflector, and the opposite end leading to the pyrometer. In order to get accurate measurement, the lamps radiation at the measured wavelength (i.e., stray radiation), as defined below must be prevented, in so far as possible, from reaching the optical fiber. A common method to do this is to place the fiber inside a cylindrical cavity with the wafer on the upper part of the cavity. The wafer lies on an opaque ring, which in turn lies on an opaque cylindrical rim. In this method, however, a large area of the wafer edge is exposed to the cavity wall, and a substantial amount of heat is lost through this exposure. As a result, there is a significant difference between the wafer center temperature and edge temperature, thereby aggravating the temperature non-uniformity problem. A quartz rim, which has low thermal conductivity, produces a better result than a metal rim, but does not eliminate the problem.